1. Technical Field
The technology described herein is generally related to the field of integrated circuits (“IC”) and, more particularly to operational amplifier circuits.
2. Description of Related Art
Two-stage complementary-metal-oxide-silicon (“CMOS”) operational amplifier (“op-amp”) circuits are ubiquitous in electronic circuit design, providing relatively high voltage gain, very high input impedance, very low output impedance, and good rejection of common-mode signals (two signal voltages of the same phase, frequency and amplitude on the inputs). One class of CMOS op-amp circuits has a differential input and a single output. FIG. 1A (Prior Art) illustrates a basic, two-stage, differential op-amp. In CMOS IC implementations, two or more differential amplifier stages are used where the gain of each stage is frequency dependent; the response of a multistage op-amp is a composite of the individual responses of the internal stages.
One problem with two-stage CMOS op-amp circuits is an inability to both source and sink a large current to the output. For example, consider a CMOS op-amp where the first stage, input, devices are p-channel metal-oxide-silicon field-effect-transistors (“MOSFET”) and the second stage consists of a p-channel pull-up device that provides a constant bias current and an n-channel pull-down device in a common-source gain configuration. As such, the current that can be sourced from the positive power supply to the output is limited to the bias current in the p-channel device. The current that can be sunk from the output to the negative power supply (or ground) is greater, due to the gain of the common-source configuration. Conversely, an op-amp with n-channel inputs can source large currents but can only sink up to the bias current in the output stage. In general it is undesirable to increase the output current capability by increasing the bias currents as that would lead to large standby mode power dissipation.
Common-mode feedback has been used in an operational amplifier having differential inputs and differential outputs wherein a predetermined common-mode output voltage independent of common-mode input voltage and input voltage variation is provided. U.S. Pat. No. 4,573,020, Feb. 25, 1886, by Whatley, for a FULLY DIFFERENTIAL OPERATIONAL AMPLIFIER WITH D.C. COMMON-MODE FEEDBACK, uses D.C. common-mode feedback to provide a common-mode output voltage of the differential operational amplifier.